1. Field of the Invention
The present invention relates to a fixing apparatus and an image forming apparatus that incorporates the fixing apparatus.
2. Description of the Related Art
Conventional image forming apparatus include printers, copying machines, facsimile machines, and multifunction peripherals. For example, an electrophotographic printer includes a charging roller, a photoconductive drum, a light emitting diode (LED) head, a developing roller, and a transfer roller. The charging roller uniformly charges the surface of the photoconductive drum. The LED head illuminates the charged surface of the photoconductive drum to form an electrostatic latent image in accordance with print data. The developing roller supplies a thin layer of toner to the electrostatic latent image as the photoconductive drum rotates, thereby developing the electrostatic latent image with the toner into a toner image. The transfer roller transfers the toner image onto print paper. After transfer, the print paper passes through a fixing unit so that the toner image on the print paper is fixed into a permanent image by heat and pressure. The print paper is then discharged onto a stacker.
One such printer is disclosed in Japanese Patent Application No. 2005-275371 and employs a belt-nip fixing unit in which a pressure pad is pressed against a fixing roller with an endless belt sandwiched between the pressure pad and the fixing roller.
FIG. 12 is a cross-sectional view of a conventional belt-nip fixing unit 101.
The fixing unit 101 includes a pressure belt assembly 102 and a fixing roller 103. The pressure belt assembly 102 includes a pressure roller 104, a pressure belt 105, a pressure pad 106, a spring 107, and a holder 108. The spring 107 urges the pressure pad 106 against the fixing roller 103 with the pressure belt 105 sandwiched between the pressure pad 106 and the fixing roller 103. The holder 108 holds the pressure pad 106 in position. The pressure pad 106 is supported such that the pressure pad 106 is slidable on the inner surface of the pressure belt 105 and can extend and retract within the holder 108 relative to the fixing roller 103. A heater 109 is disposed inside the fixing roller 103.
A nip N3 is formed between the fixing roller 103 and the pressure belt assembly 102. When the print paper carrying a toner image thereon passes through the nip N3, the toner image is fused by heat and is pressed by the pressure belt assembly 102, thereby being fixed into the print paper.
The conventional fixing unit 101 suffers from a problem in that when the pressure belt 105 runs, a friction resistance is developed between the inner surface of the pressure belt 105 and the upper surface of the pressure pad 106, and in that the friction resistance may cause the pressure pad 106 to decline, vibrate, or rattle. This may result in seizure of the pressure pad 106 within the holder 108, the pressure pad 106 becoming unable to reliably urge the pressure belt 105 against the fixing roller 103. The unstable urging force of the pressure pad 106 causes poor print quality such as disturbance, shifting, uneven quality of image or increases the load on the fixing motor (not shown) that drives the fixing roller 103 in rotation, or may cause complete stoppage of the fixing unit 101.